Avalon look and feel in XP v/s Longhorn
microsoft.public.windows.developer.winfx.avalonnewsgroup question on how Avalon on XP differs from that on Longhorn. His response is reproduced here.
From a developer point of view, the APIs and functionality exposed by Avalon do not change between XP and Longhorn.
For example, if you have the same graphics card, you will be able to get the same Avalon based experience within your application/content in both versions of the operating system. If you don't have equivalent/same graphics card, you will likely see a difference in rendering performance, rendering quality, or both.
The Longhorn end user experience (window adornment/glass, desktop interaction, default shape/color of controls, etc - the "theme") is specific to Longhorn and you will not get it on XP (whether the applications are Avalon based or not). The Longhorn desktop experience (the Desktop Window Manager), is built on Avalon.
Avalon controls, like Win32 controls, respond to the system theme, so that your application, by default, matches the look of other applications and desktop on the machine. You, as a software developer, are able to override this and provide any look to your application that you desire, and you can even expose to your end users the ability to select different looks/skins. The styling and graphics infrastructure make this fairly easy. The power and flexibility is there, at your disposal.
In relation to Avalon, one big benefit in Longhorn is the Longhorn Display Driver Model, the hardware abstraction layer under Direct3D. LDDM will provide greater stability and scalability in relation to graphics, especially in the case of multiple applications making use of the graphics card. Another benefit is the Next Generation Print Path drivers, which natively support the Avalon rendering model. Both of these will automatically benefit all Avalon applications and content running on Longhorn.